The present invention relates to an adhesively fixing oxygen-absorbing label, comprising an oxygen-absorbing sheet made of a resin composition blended with an oxygen-absorbing composition, and more particularly, to an oxygen-absorbing label having a water-resistant air-permeable layer for improving its water resistance required when applied to the preservation of foods.
Conventionally, so-called oxygen-absorbing packages using oxygen absorbents have been widely used in order to realize oxygen-free packaging of oxygen-susceptible products such as foods and drugs, thereby maintaining good qualities thereof by preventing the deterioration in quality due to oxidation and the putrefaction due to breeding of microorganisms. Oxygen-absorbing packages are ordinary used in the form of an air-permeable small bag filled with granular or powdery oxygen absorbents. There have been recently developed an oxygen-absorbing sheet having an excellent oxygen removing capability which comprises, as an oxygen absorber, an oxygen-absorbing resin sheet produced by molding a thermoplastic resin blended with powdery oxygen absorbents such as iron powder-based oxygen absorbents into a sheet shape (Japanese Patent Application Laid-Open No. 2-72851).
The oxygen-absorbing sheet may be made into a multi-layer structure by covering an oxygen-absorbing sheet containing oxygen absorbents fixed in a thermoplastic resin with a film or sheet. The oxygen-absorbing sheet is suitably applied to an automatic labeling machine as compared to the small bag oxygen absorbents because the oxygen-absorbing sheet is free from leakage of the powdery oxygen absorbents due to rupture of the bag and can be made into a thin flat sheet with a uniform thickness. In view of the configuration and multi-layer structure of the oxygen-absorbing sheet, there have been proposed adhesive oxygen absorbers having a release adhesive layer on one surface of the oxygen-absorbing sheet. For example, Japanese Patent Application Laid-Open Nos. 7-219430 and 10-290915 propose oxygen-absorbing labels of laminated structures successively comprising a release layer, a base layer having a release adhesive layer, an oxygen-absorbing layer and an air-permeable layer in which the air-permeable layer is directly bonded to the base layer at portions surrounding an outer periphery of the oxygen-absorbing layer so as not to expose the oxygen-absorbing layer outside.
However, the conventional oxygen-absorbing labels containing iron as a main oxygen absorbent are usually provided at its surface contacting with products to be preserved with an air-permeable layer made of a porous material subjected to pore-forming procedure such as needle-perforation and stretching in order to ensure good oxygen absorbability by enhancing the air permeability. Therefore, when the oxygen-absorbing labels are used in packages for foods containing a large amount of water, the water enters into the inside of the oxygen-absorbing sheet through pores of the air-permeable layer under certain conditions, thereby causing undesired dissolution of the oxygen-absorbing components and rusts. The dissolved oxygen-absorbing components and rusts may ooze to the outer surface of the oxygen-absorbing label, resulting in deteriorated appearance of the packages.
In order to solve these problems, it is known that a non-porous film is used as the air-permeable layer to allow oxygen to permeate from the end surface of the film into the oxygen absorber, thereby preventing the rusts from oozing to the surface. However, in a water-requiring oxygen-absorbing sheet containing iron as a main oxygen absorbent, water inevitable for promoting the oxygen removal, together with oxygen, should be allowed to penetrate from the end surface, resulting in low oxygen-absorbing rate.
An object of the present invention is to provide an oxygen-absorbing label that is free from oozing of oxygen-absorbing components and rusts to the outer surface of the oxygen-absorbing label even when applied to foods containing a large amount of water.
As the result of extensive studies in view of the above object, the inventors have found that the above object can be achieved by an oxygen-absorbing label having, as an air-permeable layer, a laminated film comprising a water-resistant moisture-permeable resin film having a water pressure resistance of 2,000 mmH2O or higher and a moisture permeability of 1,000 g/m2/24 h or higher and a microporous film, without causing a substantial decrease in oxygen-absorbing rate. The present invention has been accomplished on the basis of this finding.
Thus, the present invention provides an oxygen-absorbing label exhibiting an improved water resistance, which has a laminated structure successively comprising a base layer having a release adhesive layer on its surface, an oxygen-absorbing sheet made from a oxygen-absorbing resin comprising a thermoplastic resin and an oxygen absorbent blended therein, and an air-permeable layer, wherein the air-permeable layer is bonded to the base layer at a portion surrounding an outer periphery of the oxygen-absorbing sheet, and the air-permeable layer comprises (A) a water-resistant moisture-permeable resin film having a water pressure resistance of 2,000 mmH2O or higher and a moisture permeability of 1,000 g/M2/24 h or higher and (B) a microporous film.